My long-range objective is to understand the mechanisms that produce smooth eye movement appropriate to stabilize retinal images. The vestubulo-ocular reflex (VOR) stabilizes images of stationary objects by generating compensatory eye movements opposite in direction and nearly equal in amplitude to head movement. Smooth pursuit stabilizes images of small moving objects by generating slow tracking eye movements. The oculomotor system provides a model in which rigorous techniques can be used to study brain function in behaving monkeys. Our findings are therefore relevant to many forms of brain disease. In addition, smooth eye movements involve structures near the junction of the brainstem and cerebellum (pontine angle); a complete understanding of the mechanisms of smooth eye movement would facilitate early diagnosis and treatment and a better understanding of tumors and diseases that commonly affect this brain region. Four specific projects are proposed, all to be carried out in rhesus monkeys that receive liquid rewards for tracking a small, moveable target. 1) Psychophysical experiments will measure the smooth eye movement responses to a variety of precisely controlled retinal stimuli. The aim of this project is to determine what retinal inputs can affect pursuit and what inputs are actually used in normal tracking. 2) Single cell recordings will assess the discharge of cells in the vestibular nuclei and the nucleus prepositus hypoglossi during pursuit and the VOR. Electrical stimulation will be applied to the cerebellar flocculus and vestibular nerves to determine the connections of these same cells. The aim of this project is to identify pathways involved in pursuit and to determine the functional properties of cells within them. 3) Eye movement recordings will be made during the short-term interactions of visual and vestibular inputs, to investigate the pursuit exhibited during active head movements. Unexpected, transient perturbations will be applied to determine the gain and latency of pathways that are active at different times during the tracking maneuver. 4) Neurophysiological experiments will investigate the site of neural changes underlying long-term visual modifications of the VOR. The specific aim of this project is to determine whether the changes are in the disynapic pathways from the vestibular nerve to the extra-ocular motoneurons, and to further localize the synapse across which changes occur.